TODO
* Prevent Transformer::Loader#{key,attributes,object} to be overriden by mapping definitions
* Prevent Transformer::Dumper#{key,dump} to be overriden by mapping definitions
* Rename Attribute into Mapping
* Rename AttributeSet into MappingSet
* Merge this project with session?
* Remove runtme dep on veritas, and virtus once support gem is born.
* Make AttributeSet also Immutable
* Add class_method/new_spec be_frouen for all frozen objects